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Leaking PVB

I have a Watts 800M4FR pressure vacuum breaker. When I turned it on this year it started leaking from the top, apparently between the vent float and the bonnet. Information I found online suggested that I replace the faulty parts, so I did. In fact, I bought the full replacement kit and replaced every part in it, but it still continued leaking between the NEW vent float and bonnet.

I rebuilt it two more times, even tried using the sealant paste that the parts come with between the vent float and bonnet, but it still leaks. Here's a video: http://www.youtube.com/watch?v=O-wfL-JYRWw Any ideas? Thanks.

This post has been edited 1 times, last edit by "mrfixit" (Jun 25th 2013, 9:54pm)

I can't say I've seen one do what your video's showing after being repaired so I'm grasping at straws here.
Since you have the same issue even after installing new parts I'm thinking maybe the issue is elsewhere.
Has there been a drop in water pressure?
Do the sprinklers have full pressure?
I've seen ball valves break where it looks like they're open because the handle is inline with the pipe but the ball inside only partially opened.
Is there a pressure regulator on the system?
Is there another shut off valve that's not on all the way?
Maybe you bought faulty parts.

I'm not sure if the water pressure is right. It appears to be working properly, but I suppose it might be a bit low.

The problem I see with the partially-open valve theory is that the PVB still leaks even when the sprinklers are off. My physics background leads me to think that the pressure in the PVB would equal the pressure from the main water source if the water isn't flowing, as long as the valves are at least partially open. Does that sound right?

It's hard to imagine how freeze damage could cause a leak between brand new parts, so I think your faulty part theory sounds most likely to me. Maybe I'll try parts from another vendor and if that doesn't work, hire a pro.